n, and they were made circular by
placing one horse-shoe tile upon another. It would be difficult to
invent a weaker conduit. On re-drainage, innumerable instances
were found in which the upper tile was broken through the crown,
and had dropped into the lower. Next came the D form, tile and
sole in one, and much reduced in size--a great advance; and when
some skillful operator had laid this tile bottom upwards we were
evidently on the eve of pipes. For the D tile a round pipe moulded
with a flat-bottomed solid sole is now generally substituted, and
is an improvement; but is not equal to pipes and collars, nor
generally cheaper than they are."

[Illustration: Fig. 14 - SOLE TILE.]

Fig. 14 - SOLE TILE.

One chief objection to the _Sole-tiles_ is, that, in the drying which they
undergo, preparatory to the burning, the upper side is contracted, by the
more rapid drying, and they often require to be trimmed off with a hatchet
before they will form even tolerable joints; another is, that they cannot
be laid with collars, which form a joint so perfect and so secure, that
their use, in the smaller drains, should be considered indispensable.

[Illustration: Fig. 15 - DOUBLE-SOLE TILE.]

Fig. 15 - DOUBLE-SOLE TILE.

The _double-sole tiles_, which can be laid either side up give a much
better joint, but they are so heavy as to make the cost of transporation
considerably greater. They are also open to the grave objection that they
cannot be fitted with collars.

Experience, in both public and private works in this country, and the
cumulative testimony of English and French engineers, have demonstrated
that the only tile which it is economical to use, is the _best_ that can
be found, and that the best,--much the best--thus far invented, is the
"pipe, or round tile, and collar,"--and these are unhesitatingly
recommended for use in all cases. Round tiles of small sizes should not be
laid without collars, as the ability to use these constitutes their chief
advantage; holding them perfectly in place, preventing the rattling in of
loose dirt in laying, and giving twice the space for the entrance of water
at the joints. A chief advantage of the larger sizes is, that they may be
laid on any side and thus made to fit closely. The usual sizes of these
tiles are 1-1/4 inches, 2-1/4 inches, and 3-1/2 inches i